Study of Efficacy and Safety of MIJ821 in Addition to Comprehensive Standard of Care on the Rapid Reduction of Symptoms of Major Depressive Disorder in Subjects Who Have Suicidal Ideation With Intent

Inclusion Criteria: 1. Signed informed consent must be obtained prior to participation in the study 2. Male and female participants, 18 to 65 years of age (inclusive) at screening 3. DSM-5 defined major depressive disorder (MDD) with a current major depressive episode (MDE) without psychotic features at the time of screening based upon clinical assessment and confirmed by the Mini International Neuropsychiatric Interview (M.I.N.I.) assessed at Screening 4. Participants must have current suicidal ideation with intent, confirmed by a "Yes" response to Question B3 AND either Question B10 or Question B11 obtained from the M.I.N.I., assessed at Screening 5. Current suicidal ideation with intent, confirmed by "Yes" response to Question 3 AND either Question 9 or Question 10 obtained from the SSTS at Baseline 6. Montgomery-Åsberg Depression Rating Scale (MADRS) score \> 28 at Screening and before randomization on Day 1 7. Participants must agree to receive pharmacological standard of care treatment to treat their MDD (as determined by the treating physician(s) based on clinical judgement and local treatment guidelines) during the trial duration 8. In the physician's opinion, acute psychiatric hospitalization is clinically warranted to treat the patient's condition, and the patient is either already in the hospital or agrees to be hospitalized voluntarily for the required per protocol period Exclusion Criteria: 1. Any prior or current diagnosis of bipolar disorder, MDD with psychotic features, schizophrenia, or schizoaffective disorder as obtained from M.I.N.I. at Screening 2. Patients with acute alcohol or substance use disorder or withdrawal symptoms requiring detoxification, or patients who went through detoxification treatment (inpatient or outpatient) within 1 month before Screening. 3. Participant has a current clinical diagnosis of autism, dementia, or intellectual disability 4. History of seizures. Note: childhood febrile seizures are not exclusionary 5. Participants with borderline personality disorder as obtained from M.I.N.I. at Screening. 6. Participants with suicidal ideation or behavior caused primarily by another non-MDD condition as obtained from M.I.N.I. at Screening 7. Participants taking medications prohibited by the protocol 8. Intake of the following medications/ psychotherapy: 1. Esketamine or Ketamine 2 months before Screening 2. Monoamine oxidase inhibitors (MAOIs) 14 days before Screening 3. Non-stable psychotherapy regimen and/or started less than 6 weeks before Screening 9. Any other condition (e.g. known liver disease/liver dysfunction, active malignancy, etc.) which in the opinion of the investigator would put the safety of the participant at risk, impede compliance or hinder completion of the study.